1. Field of the Invention
The present invention relates in general to a numerical control system for a machine too, and particularly to a numerical control machine tool capable of immediately returning an NC drive axis to a machine origin in case of emergency.
2. Description of the Prior Art
A dangerous or abnormal situation often happens when a numerical control machine tool is controlled in accordance with a numerical control program. In order to obviate such a dangerous or abnormal situation, an emergency stop button is depressed by an operator to discontinue the operation of the machine tool immediately. In this emergency stop state, each of NC (numerical control) drive axes of the machine tool is usually away from a machine origin thereof, in which state it is impossible to restart the machine tool in accordance with the numerical control program. More importantly, the emergency stop button, when depressed, breaks power supply to every circuit component of the system, and data which is stored in the numerical controller to represent feed amounts of the NC drive axes from respective machine origins is lost in the event of the emergency stop. This makes it impossible to return each of the NC drive axes to the machine origin thereof in an automatic mode in response to the depression of the emergency stop button.
In a prior art numerical control machine tool system, in order to set the system for a subsequent restarting, the operational mode of a numerical controller used therein is manually changed from an automatic continuous mode to a jog mode. The states of NC drive axes are confirmed by an operator, and individual manipulations depending upon the confirmed states of the NC drive axes are performed to return the NC drive axes to respective machine origins through jog feeds. Otherwise, where the states of the NC drive axes permit, an automatic origin returning command is manually input and is executed by the numerical controller thereby to automatically return the NC drive axes to the respective machine origins.
In a flexible transfer machine including a plurality of numerical control machine tools connected in series by a transfer device, when an emergency stop is effected, the states of every machine tool must be confirmed and various manipulations appropriate to each of the confirmed states must be performed to return NC drive axes of each machine tool to respective machine origins. The restarting of the transfer machine in an automatic continuous mode is impossible without operator's such various manipulations. For this reason, a long period of time is spent from the occurrence of an emergency stop until the restarting of the transfer machine in the automatic continuous mode, and the operator is required to do complicated manipulations for the machine tools. That is, the discontinuation by an emergency stop of a numerical control operation needs operator's laborious works and a long period of time for restarting of the operation.